1. Field of the Invention
The present invention is directed to a method for localizing at least one focal lesion in a biological tissue section, with the lesion exhibiting an electrical property different from the tissue section, and with the electrical property in the tissue section being essentially constant.
2. Description of the Prior Art
A method of the above type is disclosed by PCT Application WO99/48422. For imaging by means of electrical impedance measurement, electrical currents are impressed and/or electrical voltages are applied to an examination subject at one or more locations therein. Voltages that arise due to the impressed currents are measured using M electrodes (M≧1) that are brought into electrical contact with the tissue section under examination at one or more locations. Alternatively, currents that are established due to the applied voltages are measured either exclusively or additionally as well. The voltages and/or currents are determined by the electrical properties of the subject (described, for example, by the complex conductivity in the mathematical sense). Measured data at m different locations thus are obtained.
The electrical conductivity is composed of a d.c. component and frequency-dependent polarization current components. By analogy to alternating current technology, the electrical conductivity is accordingly mathematically described as a complex quantity.
For example, 64 or 256 time-dependent current values can be measured at present at the surface of the female breast by means of 8×8 or 16×16 regularly arranged electrodes using an apparatus of the TransScan Company that is distributed under the name TS2000. The measurement area amounts to approximately 7.9×7.9 cm2. The measured current values arise due to an alternating voltage between the measuring electrodes and a reference electrode at the contra-lateral hand. The measured data, magnitude and phase of the current are individually converted into conductance values and capacitance values are presented in conformity with the two-dimensional electrode arrangement.
When focal lesions that, for example, exhibit a higher electrical conductivity than the surrounding tissue are situated in the tissue section under the electrode arrangement, then—for example in the case of current measurements—higher current values are measured in the electrodes lying immediately thereabove. Such a lesion is visible as a peak in the two-dimensional measured data presentation. The peak amplitude and the peak width are dependent on the size and depth of the lesion and on the difference in conductivity between the lesion and the surrounding tissue.